The present invention relates to a method of producing a nanocrystalline alloy having an extremely high permeability, which is used in various magnetic parts of transformers, choke coils, etc.
As a material for a magnetic core of a common-mode choke coil used in a noise filter, a pulse transformer, etc., a high permeability material having excellent high-frequency properties such as ferrite, amorphous alloy, etc. has been used. The material for a magnetic core of common-mode choke coil used in a noise filter (line filter) is further required to have an excellent pulse attenuation characteristics for preventing disordered operation of an apparatus equipped therewith due to high-voltage pulse noise caused by thunder, a large inverter, etc. However, since the ferrite material, which has been conventionally used, is low in saturation magnetic flux density, it easily reaches a magnetically-saturated state. This means that a small-sized magnetic core made of the ferrite material cannot show a sufficient efficiency to fail to meet the above requirements. Therefore, a large-sized core is necessary for obtaining a high efficiency when ferrite is used as the core material.
An Fe-based amorphous alloy has a high saturation magnetic flux density and shows, with respect to a high-voltage pulse noise, excellent attenuation characteristics as compared with the ferrite material. However, since the permeability of the Fe-based amorphous alloy is lower than that of a Co-based amorphous alloy, it shows insufficient attenuation to a low-voltage noise. In addition, the Fe-based amorphous alloy shows a remarkably large magnetostriction. This invites further problems such as alteration in its properties caused by a resonance with vibration due to magnetostriction at a certain frequency, and beating of the magnetic core when a current having audio frequency component flows through a coil.
A Co-based amorphous alloy shows a large attenuation to low-voltage noise due to its high permeability. However, since the saturation magnetic flux density is lower than 1 T, the Co-based amorphous alloy shows poor attenuation to high-voltage pulse noise as compared with an Fe-based amorphous alloy. Further, the Co-based amorphous alloy of a high permeability is lacking in reliability due to its significant deterioration of properties with time, in particular under environment of a high ambient temperature.
A material for magnetic core of a pulse transformer which is used in an interface to the ISDN (Integrated Services Digital Network) is required to have a high permeability, in particular, at around 20 kHz and a high stability of properties against temperature. In some applied use, a material showing a flat B-H loop having a low remanence ratio is required, however, a material having a specific initial permeability of 100000 or more has been difficult to be obtained. Recently, the application of the pulse transformer to card-type interface has come to be considered. This requires a small-sized and thin pulse transformer which satisfies the restriction of an inductance of 20 mH or more at 20 kHz. To meet such requirement, the material is necessary to have a still more higher permeability. Further, a material showing a flat B-H loop having a low remanence ratio and having a stability in permeability is also required for a high fidelity transmission. However, ferrite and an Fe-based amorphous alloy cannot satisfy the above demand due to their low permeability. Ferrite has another demerit that the permeability thereof largely depends on temperature, in particular, it is drastically lowered at a temperature lower than room temperature. Although a high permeability can be obtained, the Co-based amorphous alloy shows a large change with time in its permeability at a high ambient temperature and is expensive, therefore, the application of such an alloy to a wide use is restricted.
A material having a high permeability is further required in an electric sensor used in electrical leak alarm, etc. and a magnetic sensor in view of a small size and a high sensitivity. Further, a highly permeable material showing a flat B-H loop having a low remanence ratio and having a stability in permeability is required for a linear output.
A nanocrystalline alloy (fine crystalline alloy) has been used to produce a magnetic core of common-mode choke coils, high-frequency transformers, electrical leak alarms, pulse transformers, etc. because of its excellent soft magnetic properties. Typical examples for such a nanocrystalline alloy are disclosed in U.S. Pat. No. 4,881,989 and JP-A-1-242755. The nanocrystalline alloy known in the art has been generally produced by subjecting an amorphous alloy obtained by quenching a molten or vaporized alloy to a heat treatment for forming fine crystals. A method for quenching a molten metal may include a single roll method, a twin roll method, a centrifugal quenching method, a rotation spinning method, an atomization method, a cavitation method, etc. A method for quenching a vaporized metal may include a sputtering method, a vapor deposition method, an ion plating method, etc. The nanocrystalline alloy is produced by finely crystallizing an amorphous alloy produced by the above method, and is known to have, contrary to amorphous alloys, a good heat stability as well as a high saturation magnetic flux density, a low magnetostriction, and a good soft magnetic property. The nanocrystalline alloy is also known to show a little change with time in its properties and have a good temperature stability. Specifically, the Fe-based nanocrystalline alloy disclosed in U.S. Pat. No. 4,881,989 is described t o have a high permeability and a low magnetic core loss, and therefore, suitable for the use mentioned above.
As mentioned above, a magnetic core for a common-mode choke used in a noise filter, a pulse transformer for use in ISDN, etc. are required to have a high specific permeability. U.S. Pat. No. 4,881,989 disclose heat-treating an amorphous alloy at 450.degree.-700.degree. C. for 5 minutes to 24 hours. However, a nanocrystalline alloy produced by the conventional heat treatment method cannot attain a high specific initial permeability exceeding 100000.